I have more than 2 wireless devices - my BluRay player is WiFi, and I have a WiFi enabled printer and external hard drive. (However, to my knowledge, none of those are Calibre compatible.) I have also had friends over who have used my WiFi for their computer/phone for a weekend or whatever.

Thanks again for all your help!

Those explains the spread
I was suspecting your (Calibre host) IP got changed during a lease renewal